Researchers at The University of Texas at Austin have developed a simple scaling theory（标度理论） to estimate gas production from hydraulically（水压的） fractured wells in the Barnett Shale. The method is intended to help the energy industry accurately identify low- and high-producing horizontal wells, as well as accurately predict how long it will take for gas reserves to deplete in the wells. Using historical data from horizontal wells in the Barnett Shale formation in North Texas, Tad Patzek, professor and chair in the Department of Petroleum and Geosystems Engineering in the Cockrell School of Engineering; Michael Marder, professor of physics in the College of Natural Sciences; and Frank Male, a graduate student in physics, used a simple physics theory to model the rate at which production from the wells declines over time, known as the "decline curve."

They describe their new model of the decline curve in the paper "Gas production in the Barnett Shale obeys a simple scaling theory," published this week in the Proceedings of the National Academy of Sciences. To test their theory, the researchers analyzed 10 years of gas production data from the Barnett Shale licensed to the university by IHS CERA, a provider of global market and economic information.

On average, they found that gas production in individual wells begins declining after about five years of production. They also found that wells generally produce less gas than predicted under previous, theoretical models and that production can be increased if hydrofractures connected better to the natural fractures in the rock.

The team's estimates were an instrumental part of the comprehensive assessment of Barnett Shale reserves funded by the Alfred P. Sloan Foundation and issued earlier this year by the Bureau of Economic Geology at UT Austin.

Until now, estimates of shale gas production have primarily relied on models established for conventional oil and gas wells, which behave differently from the horizontal wells in gas-rich shales.

The researchers estimate the ultimate gas recovery from a sample of 8,294 horizontal wells in the Barnett Shale will be between 10 trillion and 20 trillion standard cubic feet (scf) during the lifetime of the wells. The study's well sample is made up of about half of the 15,000 existing wells in the Barnett Shale, the geological formation outside Fort Worth that offers the longest production history for hydrofractured horizontal wells in the world.

"With our model at hand, you can better predict how much gas volume is left and how long it will take until that volume will be depleted," Patzek said. "We are able to match historical production and predict future production of thousands of horizontal gas wells using this scaling theory."

"The contributions of shale gas to the U.S. economy are so enormous that even small corrections to production estimates are of great practical significance," Patzek said.

The researchers were surprised by how all of the wells they analyzed adhere to that simple scaling curve.

"By analyzing the basic physics underlying gas recovery from hydrofractured wells, we calculated a single curve that should describe how much gas comes out over time, and we showed that production from thousands of wells follows this curve," Marder said.

Patzek adds: "We are able to predict when the decline will begin. Once decline sets in, gas production goes down rapidly."